This sauce would also be fantastic on roasted chicken.
After cooking your steak (or other meat), drain off excess fat from the pan. Place the skillet over medium-high heat, and when it gets hot, add a generous spoonful of whole grain mustard. Pour in about half a cup of white wine, and bring to a simmer. Stir constantly with a rubber spatula or wooden spoon, scraping any crispy bits off the bottom of the skillet. Simmer until reduced by half. Add a generous spoonful of crème fraîche and stir briskly to combine. Return to a simmer. Remove from the heat and stir in a handful of minced parsley and a pinch of salt.